Polyimides constitute a class of valuable polymers being characterized by thermal stability, inert character, usual insolubility in even strong solvents, and high T.sub.g, among others Their precursors are usually polyamic acids, which may take the final imidized form either by thermal or by chemical treatment.
Polyimides have always found a large number of applications requiring the aforementioned characteristics in numerous industries, and recently their applications have started increasing dramatically in electronic devices, especially as dielectrics. With continuously escalating sophistication in such devices, the demands on the properties and the property control are becoming rather vexatious.
Especially for the electronics industry, improvements of polyimides are needed in forming tough, pin-hole free coatings, having lower dielectric constant, lower linear coefficient of thermal expansion, and lower moisture absorption, among others. It is not usually possible to maximize all properties, since many of them are antagonistic. Thus, only a compromised solution has so far been achieved by at least partially sacrificing one or more of these properties in order to maximize a desired one.
It has now been found that polyimides based on 9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and a class of benzidine derivatives provide compositions which may be used to form dielectric films for electronic circuits characterized by high thermal stability, low linear coefficient of thermal expansion, low dielectric constant, and low water absorption.
Japanese Patent Application Publication Kokai Hei 2-60933 (Masaki Ishisawa et al., Pub. Date: Mar. 1, 1990) discloses certain compositions of polyimides containing a benzidine derivative and derivatives containing fluorochains. However, one of the disadvantages of these compositions is the fact that the fluorochains contain adjacent carbon atoms having hydrogen and fluorine atoms, and are subject to premature thermal decomposition, due to dehydrofluorination. This publication does not recognize the importance of low water absorption, as this property is not even mentioned. Furthermore, this publication does not mention, suggest, or imply the combination of 9-aryl-9-(perfluoroalkyl)xanthene-2,3,6,7-dianhydride or 9,9'-bis(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and benzidine derivatives for achieving the advantages of the present invention.